1. Field of the Invention
The present invention relates to a ball circulation groove structure of a ball nut.
2. Description of the Related Art
As a ball circulation groove structure of a ball nut in which the ball nut, a thread shaft, and a ball are combined, and a thread shaft is linearly moved on the basis of a rotation of the ball nut, there is a structure in which a deflector is formed by a resin in a hole penetrating from an inner surface of the ball nut to an outer surface, adjacent spiral grooves of the ball nut are joined by an S-shaped return groove of the deflector, and a ball rolling along the spiral groove of the thread shaft is guided along a peak portion formed between the adjacent spiral grooves of the thread shaft by the return groove of the deflector and is returned to the adjacent spiral groove of the thread shaft over the peak portion for recirculation, as described in Japanese Patent Publication No. 61-45096(patent document 1).
In the ball circulation groove structure of the ball nut described in the patent document 1, the deflector is formed by filling a molten resin in a hole of the steel ball nut. Accordingly, the resin deflector contracts within the hole of the ball nut at a time of a low temperature, on the basis of a difference of a coefficient of linear expansion between the ball nut and the deflector. Therefore, the deflector rattles within the hole of the balll nut, a step is generated in a connection portion between the return groove of the deflector and the spiral groove of the ball nut, and there is a risk that the step obstructs smooth rolling of the ball nut.